This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Carbon fixation and productivity of many ecosystems are limited by the availability of nitrogen, and N2 fixation by soil bacteria is an important source of nitrogen to terrestrial systems. A potentially important control on nitrogen fixation in soils and leaf litter is the chemical speciation and bioavailability of V, Mo, and Fe because they are required metal cofactors in nitrogenase, the enzyme responsible for biological N2 fixation. We are combining a vanadium K-edge XANES study of V oxidation state and coordination geometry with other experiments using NMR and EPR to (1) characterize the speciation and redox transformation of V in leaf litter and soils, and (2) elucidate how the chemical speciation of V in these environments controls its bioavailability to asymbiotic diazotrophs, and thus how V speciation influences the distribution and activity of free living nitrogen fixers in terrestrial ecosystems.